The major objective of this research is to investigate the relationship of Lyme disease spirochete (LDS), Borrelia burgdorferi, to its known (Ixodes pacificus) and potential tick vectors, vertebrate hosts, and environmental factors in the western USA. Such knowledge will help elucidate the biotic/abiotic factors that maintain and distribute the LDS in this newly recognized endemic region, and may shed light on factors contributing to the recent emergence and rapid dissemination of the agent in eastern USA. Organotropisms of the LDS in its tick vector(s) and tick infection rates from different areas will be determined by dissecting adult ticks individually and examining their tissues by direct immunofluorescence. Isolation will be attempted by inoculating infected tick suspensions into modified Kelly's medium. The prevalence and intensity of I. pacificus infestation of its primary mammalian and reptilian hosts will be evaluated seasonally, and LDS-infection rates in these vertebrates and their associated ticks will be determined by standard microbiological procedures. Transmission routes within tick populations (i.e., transovarial, transstadial) and the efficiency of these processes for maintaining the LDS will be evaluated by feeding wild-caught ticks on laboratory animals and subsequently examining recently molted ticks or the progeny of ovarially-infected females for presence of spirochetes. The vector specificity of the LDS for l. pacificus will be determined by inspecting various tissues of several hundred individuals of 2 other human-biting ticks for Borrelia. Tick feeding and animal inoculation experiments will be conducted with laboratory animals and wild rodents to discover a suitable vertebrate host for laboratory transmission studies; thick and thin blood smears from experimentally-infected animals will be examined daily for 28 days. Short term ecological/epidemiological studies will be undertaken in human case-acquisition areas to find out if intense foci of infection exist in case vs. non-case contraction localities, and to determine if the chain of infection of the LDS is the same in different geographical areas. Finally, host-seeking of adult I. pacificus with respect to environmental, seasonal, and temporal factors, and LDS-infection rates will be determined experimentally in the field in specially-constructed "tick arenas" and by systematically flagging native vegetation throughout the diurnal cycle. Multivariate statistical analyses will be used to evaluate the effect of meteorological factors on adult tick behavior.